The present invention relates to integrated circuits, and more particularly, the present invention relates to testing integrated circuits.
Integrated circuits are designed to operate under a variety of environmental conditions. For example, integrated circuits are designed to operate over a range of temperatures. To ensure that an integrated circuit operates correctly over a particular range of temperatures, the integrated circuit is tested at different temperatures within the particular range of temperatures.
FIG. 1 shows a prior art test unit 101 for producing a constant temperature environment in which an integrated circuit can be tested. The test unit 101 includes a constant temperature air source 103, a test fixture 105, and an enclosure 107. A die 109, which includes an integrated circuit, is inserted in the test fixture 105 for electrical testing. The constant temperature air source 103 is capable of providing a stream of constant temperature air to the test fixture 105. The enclosure 107, when inserted between the test fixture 105 and the constant temperature air source 103, provides a cavity 111 in which a constant temperature environment is created. The enclosure 107 is fabricated from stacked foam rubber pads 113, 115, and 117. Each of the foam rubber pads 113, 115, and 117 has a hole such that when the foam rubber pads are stacked, as shown in FIG. 1, the holes are aligned to form the cavity 111. Each of the foam rubber pads 113, 115, and 117 also has one surface 119, 121, and 123, respectively, coated with a conductive material. At least one of the surfaces 119, 121, or 123 is attached to a ground 125.
Table 1 shows electrostatic discharge data for a grounded sheet of silicone foam charged to xc2x15000 volts in free air and ionized air and an ungrounded sheet of silicone foam charged to xc2x15000 volts in free air and in ionized air. As can be seen from Table 1, a grounded silicone foam sheet discharges in about 100 milliseconds, and an ungrounded sheet does not discharge in free air and discharges in between about 177 seconds and about 205 seconds in ionized air.
Unfortunately, several problems can occur during the operation of the test unit 101 during the testing of the die 109. First, an electrostatic charge 127 can accumulate on the surfaces 119, 121, and 123 of the enclosure 107, and an electrostatic charge 129 can accumulate on the test fixture 105. The accumulation of electrostatic charge 127 on the enclosure 107 and the accumulation of electrostatic charge 129 on the test fixture 105 can cause currents to flow in the test fixture 105. The currents flowing in the test fixture 105 may damage the die 109 and any electronic circuits (not shown) connected to the test fixture 105. The problem is particularly severe when any of the surfaces 119, 121, or 123 become ungrounded. As shown in the test data in Table 1, the decay time is between about 177 seconds and about 205 seconds for an ungrounded silicone sheet. This long decay time increases the likelihood of the die 109 becoming damaged by electrostatic discharge. Second, since the stacked foam rubber pads 113, 115, and 117 provide an enclosure having poor structural integrity, the location of the pads can change between uses of the test unit 101. A change in pad location makes it difficult to get consistent results when performing repeatability tests using the test unit 101.
For these and other reasons there is a need for the present invention.